format JSX expression children with explicit braces

[tsnobip]

Prepare JSX for future literal text by formatting expression children with explicit braces:

// Before
let x = <span> hello </span>

// After
let x = <span>{hello}</span>

Existing unbraced expressions remain valid syntax. Nested JSX elements stay unwrapped, and existing expression braces do not accumulate.

Child comments move inside the braces. For a single expression child, line wrapping also stays inside the braces, avoiding whitespace between the expression container and its tags:

let x =
  <span>{
    hello // some comment
  }</span>

Standalone comments in empty elements or between nested elements are wrapped in comment-only containers:

let x =
  <div>
    {/* before */}
    <Icon />
    {
      // after
    }
  </div>

The parser retains these comments without adding a child, preserving the distinction between zero, one, and multiple children. Bare {} still denotes an empty record expression; {{/* comment */}} keeps an empty record as a child. Line comments retain their style, including when their text contains */.

Literal text parsing and whitespace rules between sibling children remain future work. The next migration step is to reject unbraced expressions before enabling literal text, so existing variable references cannot silently become strings.

Validation:

• Formatting tests check expression and comment preservation and repeatability at five widths, including leading/trailing comments, fragments, nested elements, and attributes. Malformed containers remain parse errors.
• Parser and JSX transform tests verify unchanged child counts for React and custom JSX modules. AST bridge fixtures and generated JavaScript cover the transformation and JSX-preserve paths.

Supersedes #7863.

[chatgpt-codex-connector]

Codex Review Summary

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Review	Status	Commit	Review trigger
📝 Code Review	✅ Completed 2026-09-22T15:13:17.962035Z	6d6005b	Manual request

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[chatgpt-codex-connector]

💡 Codex Review

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Reviewed commit: a479f8349e

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[tsnobip]

@codex review

[chatgpt-codex-connector]

💡 Codex Review

Here are some automated review suggestions for this pull request.

Reviewed commit: 701313d0e0

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[codecov]

Codecov Report

❌ Patch coverage is 98.63014% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 78.58%. Comparing base (fb35986) to head (6d6005b).

Files with missing lines	Patch %	Lines
compiler/syntax/src/res_printer.ml	97.87%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #8666   +/-   ##
=======================================
  Coverage   78.57%   78.58%           
=======================================
  Files         476      476           
  Lines       64157    64172   +15     
=======================================
+ Hits        50413    50431   +18     
+ Misses      13744    13741    -3     

Files with missing lines	Coverage Δ
compiler/syntax/src/res_comments_table.ml	88.61% <100.00%> (-0.46%)	⬇️
compiler/syntax/src/res_core.ml	91.90% <100.00%> (+0.01%)	⬆️
compiler/syntax/src/res_parens.ml	87.85% <100.00%> (+2.91%)	⬆️
compiler/syntax/src/res_printer.ml	93.41% <97.87%> (+0.01%)	⬆️

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[pkg-pr-new]

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commit: 6d6005b

[github-actions]

Developer playground preview: https://rescript-lang.github.io/rescript/dev-playground/?version=pr-8666

[tsnobip]

@codex review

[chatgpt-codex-connector]

Codex Review: Didn't find any major issues. 🎉

Reviewed commit: 6d6005ba9f

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Codex can also answer questions or update the PR. Try commenting "@codex address that feedback".